Catalytic dehydrogenation processes are commonly used for the production of less saturated aromatic hydrocarbons from the dehydrogenation of alkylaromatic hydrocarbons. One commercialized application of this process is for the conversion of ethylbenzene to styrene. The catalytic dehydrogenation of ethylbenzene to produce styrene is an endothermic equilibrium-controlled reaction that also produces hydrogen.
One conventional catalytic dehydrogenation process for the production of styrene employs a series of reactors each containing a dehydrogenation catalyst. A heated feed stream of ethylbenzene is introduced to a first reactor at a desired reaction temperature and contacts the dehydrogenation catalyst forming a product mixture of styrene and hydrogen. As the feed stream and the product mixture advance through the reactor, the temperature drops because the reaction is endothermic and the rate of conversion of ethylbenzene to styrene rapidly decreases. A lower temperature intermediate effluent stream comprising styrene, hydrogen, and unreacted ethylbenzene is removed from the first reactor, heated to a desired reaction temperature, and introduced to a second reactor for additional conversion of ethylbenzene to styrene and hydrogen. This process may be repeated using one or more additional reactors to improve the product yield. Unfortunately, the overall conversion of ethylbenzene to styrene typically only reaches about 60 to 65% with the additional reactors because the reaction is equilibrium-controlled. This results in an inefficient process with large volumes of unreacted ethylbenzene that are costly to recover and recycle.
More recently, another catalytic dehydrogenation process for the production of styrene has been employed for improving the overall conversion of ethylbenzene to styrene. The process uses a series of reactors where the first reactor of the series contains a dehydrogenation catalyst as described above, and at least one additional reactor that is a multi-catalyst reactor contains both an oxidation catalyst and a dehydrogenation catalyst. In this process, oxygen is added to the intermediate effluent stream after the first reactor but before the effluent stream is introduced to the multi-catalyst reactor. Once introduced to the multi-catalyst reactor, the intermediate effluent stream contacts the oxidation catalyst burning at least a portion of the hydrogen with oxygen to heat the intermediate effluent stream to a desired reaction temperature. The heated intermediate effluent stream contacts the dehydrogenation catalyst for additional conversion of ethylbenzene to styrene and hydrogen. Because at least a portion of the hydrogen in the intermediate effluent stream has been consumed to generate heat, ideally the equilibrium-controlled reaction of ethylbenzene to styrene and hydrogen favors the product side to improve the levels of ethylbenzene conversion. Unfortunately, an improvement in the overall ethylbenzene conversion is not realized because some of the ethylbenzene and styrene burn from contact with the oxidation catalyst in the presence of oxygen forming carbon monoxide and carbon dioxide. This has a negative impact on the performance of the dehydrogenation catalyst including reducing catalyst activity and shortening catalyst life.
Accordingly, it is desirable to provide methods and apparatuses for producing styrene with improved overall ethylbenzene conversion to styrene. Moreover, it is desirable to provide methods and apparatuses for producing styrene without negatively impacting the performance of the dehydrogenation catalyst otherwise caused from forming carbon monoxide and carbon dioxide. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent Detailed Description of the Invention and the appended Claims, when taken in conjunction with the accompanying drawings and this Background of the Invention.